The invention relates to a process for imidizing copolymers of maleic anhydride with vinyl-aromatic monomers, in particular styrene, and optionally with other monomers.
These copolymers are known, for instance, from U.S. Pat. No. 3.954.722; another U.S. Pat. No. (4.167.543) foresees the addition of acrylonitrile as a third monomer. Said copolymers, marketed, for instance by the ARCO Company under the trade mark `DYLARC`, may be processed under heat, but when they are blended with other polymeric products at high temperatures (in order to obtain, for example, an impact resisting material) there are observed decomposition reactions as well as the formation of surface blisters and of cross-linkings.
The processing becomes thus extremely difficult and the look of the surfaces may be unacceptable. If the maleic anhydride is replaced by the corresponding imide, as described in British Pat. No. 1.185.544, these drawbacks disappear and there are processes which allow to obtain these copolymers starting from a pre-formed maleic imide as a starting monomer, thereby obtaining substantially equimolar copolymers (styrene:imide=1:1 by mols) or non-equimolar copolymers (with an imide content lower than 50% by mols). The maleic imide is not, however, easily available and its synthesis is rather troublesome and complicated.
Alternatively, U.S. Pat. Nos. 3.651.171; 3.840.499; 4.381.373 and German Pat. No. 3.430.802, teach to imidize a preformed copolymer in the presence of water. Moreover, U.S. Pat. No. 4.404.322 teaches that the imidization with amines (notoriously very expensive) may be carried out either in the molten state or in an organic solvent. The contents of all the above cited patents are an integrating part of the present description. The imidized copolymers thus obtained have a much higher glass transition temperature, (Tg, defined further on), with respect to the non-imidized copolymers, but the Applicant has now found that there is a much simpler and more economical process which allows to obtain copolymers imidized at a level of quality equivalent and sometimes superior.